This application claims Paris Convention priority of DE 100 41 683.7 filed Aug. 24, 2000 the complete disclosure of which is hereby incorporated by reference.
The invention concerns a superconducting magnet arrangement for generating a magnetic field in the direction of a z axis in a working volume disposed about z=0, with a magnet coil system having at least one current-carrying superconducting magnet coil, a shim device with at least one superconducting shim coil and one or several additional superconductingly closed current paths, wherein the magnetic fields in the z direction produced by induced currents through the additional current paths during operation do not exceed a magnitude of 0.1 Tesla in the working volume, and wherein the shim device generates a field which varies along the z axis with the kth power of z for an even power of k greater than 0.
A superconducting magnet arrangement with a Z2 shim device of this type is part of e.g. the NMR Magnet System 600 SB UltraShield(trademark) (company leaflet dated May 15, 1999) distributed by the company Bruker Magnetics.
U.S. Pat. No. 5,329,266 describes a superconducting magnet arrangement with an actively shielded magnet and additional superconducting current paths without a Z2 shim device.
Superconducting magnets are used for different applications, in particular for magnetic resonance methods, in which the local homogeneity of the magnetic field is usually important. One of the most demanding applications is high-resolution nuclear magnetic resonance spectroscopy (NMR spectroscopy). The basic homogeneity of the superconducting magnet can be optimized through the geometrical arrangement of the field-generating magnet coils. In demanding applications, the basic homogeneity of the magnet is usually insufficient due to deviations from the design caused by production tolerances. To compensate for residual inhomogeneities of the magnet, the magnet system is equipped with autonomous superconducting coils which compensate for field inhomogeneities having a certain geometrical symmetry in the working volume, so-called shim devices. Examples of such shim devices are so-called Zn shims which produce a field which varies along the magnet axis z as zn. The main focus of the invention is the dimensioning of superconducting Zn shims in magnet systems with active stray field shielding and additional superconducting current paths, e.g. to compensate for external field fluctuations.
The field contribution of a superconducting Zn shim in a working volume must be substantially zero in the working volume at z=0, irrespective of the current in the shim coils, thereby taking into consideration the field contributions of the shim coils themselves and also of the field change due to currents induced in the superconducting magnet and in further superconductingly closed current paths during charging of the shim device. Dimensioning of a Zn shim according to a conventional method will produce, in certain cases, an undesired shift of the magnetic field strength in the working volume at z=0 during charging of the shim device. In actively shielded magnets, this behavior is particularly clear with shim devices whose coils are distributed over different radii because the conventional methods for dimensioning superconducting shim devices treat the superconductor as a non-magnetic material. The present invention also takes into consideration that the superconductor substantially exhibits a diamagnetic material behavior in response to field fluctuations of less than 0.1 Tesla, which can e.g. occur in the magnet volume during charging of a superconducting shim device, and thereby largely expels small field fluctuations from its inner regions. This results in a redistribution of the magnetic flux of the field fluctuations in the magnet arrangement which, in turn, influences the reaction of the superconducting magnet and further superconductingly closed current paths to a current change in the shim device, since this reaction is determined by the principle of magnetic flux conservation through a closed superconducting loop.
In contrast thereto, it is the underlying purpose of the present invention to modify a superconducting Zn shim in a magnet arrangement of the above-mentioned type in as simple a manner as possible such that the shim device is correctly dimensioned while taking into consideration the diamagnetism of the superconductor such that, in particular, its field contribution in the working volume is substantially zero in the working volume at z=0, irrespective of the current in the shim coils.
This object is achieved in accordance with the invention by designing the shim device such that the variable gSeff=gSxe2x88x92gTxc2x7(Lclxe2x88x92xcex1Lcor)xe2x88x921xc2x7(L←Sclxe2x88x92xcex1L←Scor) is substantially zero if and only if the variable gSeff,cl=gSxe2x88x92gTxc2x7(Lcl)xe2x88x921xc2x7L←Scl, which would have resulted were xcex1=0, is larger than zero, in particular larger than 0.2 millitesla per ampere.
The above variables have the following definitions:
gSeff: Field contribution per ampere current of the shim device in the working volume at z=0 thereby taking into consideration the field contributions of the shim coils themselves and of the field change due to currents induced in the superconducting magnet coil system and in the further superconductingly short-circuited current paths during charging of the shim device,
xe2x88x92xcex1: average magnetic susceptibility in the volume of the magnet coil system with respect to field fluctuations which do not exceed a magnitude of 0.1 T, wherein 0 less than xcex1xe2x89xa61,
gT=(gM, gP1, . . . , gPj, . . . , gPn),
gPj: Field per ampere of a current path Pj in the working volume without the field contributions of current paths Pi for ixe2x89xa0j and of the magnet coil system and without the field contributions of the shim device,
gM: Field per ampere of the magnet coil system in the working volume without the field contributions of the additional current paths and without the field contributions of the shim device,
gS: Field per ampere of the shim device in the working volume without the field contributions of the additional current paths and of the magnet coil system,
Lcl: Matrix of inductive couplings between the magnet coil system and the additional current paths and among the additional current paths,
Lcor: Correction for the inductance matrix Lcl, which would result with complete diamagnetic expulsion of disturbing fields from the volume of the magnet coil system,
L←Scl: Vector of the inductive couplings of the shim device with the magnet coil system and the additional current paths,
L←Scor: Correction for the coupling vector L←Scl, which would result for complete diamagnetic expulsion of disturbance fields from the volume of the magnet coil system.
According to prior art, correct dimensioning of a Zn shim entails correct calculation of the field efficiencies gS, gp1, . . . , gPn and gM of the shim device, of the additional superconducting current paths and of the magnet (without taking into consideration the respective reactions of the other components) and the mutual inductive couplings among the shim device, the additional current paths and the magnet as well as all self-inductances, wherein the shim device is then designed such that the variable gSeff,cl=gSxe2x88x92gTxc2x7(Lcl)xe2x88x921xc2x7L←Scl is substantially zero. When dimensioning the shim device of an arrangement in accordance with the invention, in addition to the above-mentioned coil properties, the magnetic shielding behavior of the superconducting volume portion of the magnet coil system is also taken into consideration. For this reason, the shim device is dimensioned such that instead of the variable gSeff,cl the variable gSeff=gSxe2x88x92gTxc2x7(Lclxe2x88x92xcex1Lcor)xe2x88x921xc2x7(L←Sclxe2x88x92xcex1L←Scor) is substantially zero. The above-mentioned magnetic shielding behavior of the superconductor is present in all superconducting magnet arrangements, but only has significant effect on the variable gSeff in certain cases. The present invention is advantageous in that such magnet arrangements also satisfy the requirement for the shim device that gSeff be substantially zero.
The above-mentioned advantages of the invention are significant mainly in systems where the local homogeneity of the magnetic field in the working volume is particularly important. In one preferred embodiment, the inventive magnet arrangement is therefore part of an apparatus for high-resolution magnetic resonance spectroscopy, e.g. in the field of NMR, ICR or MRI.
In an advantageous further development of this embodiment, the magnetic resonance apparatus comprises a means for field locking the magnetic field generated in the working volume.
In an improved further development, the magnet arrangement can also comprise field modulation coils.
One embodiment of the inventive magnet arrangement is particularly preferred, wherein the superconducting magnet coil system comprises a radially inner and a radially outer coaxial coil system which are electrically connected in series, wherein these two coil systems each generate one magnetic field of mutually opposite direction along the z axis in the working volume. In such an arrangement, the diamagnetic shielding behavior of the superconductor typically has a particularly strong effect on the effective field strength in the working volume gSeff of certain shim devices in the magnet coil system.
In a further development of this embodiment, the radially inner coil system and the radially outer coil system have dipole moments approximately equal in value and opposite in sign. This is the condition for optimum suppression of the stray field of the magnet coil system. Due to the great technical importance of actively shielded magnets, the correct dimensioning of shim devices in magnets, wherein the above-mentioned magnetic shielding behavior of the superconductor in the magnet coil system has significant influence on the effective field strength in the working volume gSeff of the shim device, is particularly advantageous.
In another advantageous further development of one of the above embodiments, the magnet coil system forms a first current path which is superconductingly short-circuited during operation, and a disturbance compensation coil which is not galvanically connected to the magnet coil system is disposed coaxially to the magnet coil system and forms a further current path which is superconductingly short-circuited during operation. The presence of a disturbance compensation coil improves the temporal stability of the magnetic field in the working volume under the influence of external field fluctuations. Such a further development of an inventive magnet arrangement takes into consideration the influence of a disturbance compensation coil on the effective field strength in the working volume gSeff of the shim device.
In one further advantageous development, at least one of the additional current paths consists of a part of the magnet coil system which is bridged with a superconducting switch. Such an arrangement improves the temporal stability of the magnetic field in the working volume under the influence of external field fluctuations. Such a further development of an inventive magnet arrangement takes into consideration the influence of bridging part of the magnet coil system with a superconducting switch on the effective field strength in the working volume gSeff of the shim device.
In an advantageous further development of the inventive magnet arrangement, at least one of the additional current paths is part of a system for compensating the drift of the magnetic coil system. Such an arrangement improves the temporal stability of the magnetic field in the working volume. Such a further development of an inventive magnet arrangement takes into consideration the influence of drift compensation on the effective field strength in the working volume gSeff of the shim device.
In one further advantageous development, at least one of the additional current paths consists of a further superconducting shim device. Such an arrangement compensates for field inhomogeneities which have different symmetries. Such a further development of an inventive magnet arrangement takes into consideration the influence of the additional superconducting shim devices on the effective field strength in the working volume gSeff of the first shim device.
One particularly preferred embodiment of the inventive magnet arrangement is characterized in that the superconducting shim device produces a field in the working volume having a z2 dependence along the z axis. Such shim devices are particularly important since superconducting magnet coil systems often have a field inhomogeneity which has a z2 dependence along the z axis in the working volume.
In a particularly advantageous further development of this magnet arrangement, the superconducting shim device comprises partial coils which are wound at different radii to permit a more compact construction of the shim device. The magnetic shielding behavior of the superconductor in the magnet coil system of such an arrangement typically has a particularly strong effect on the effective field strength in the working volume gSeff of the shim device.
Another advantageous further development of an inventive magnet arrangement is characterized in that the superconducting shim device is inductively decoupled from the superconducting magnet coil system to prevent high induced currents in the shim device during charging or during a quench of the superconducting magnet coil system.
An advantageous further development of this embodiment utilizes the different polarities of the radially inner and the radially outer coil system for inductive decoupling of the magnet coil system and shim device, wherein the superconducting magnet coil system has a radially inner and a radially outer coaxial coil system which are electrically connected in series, with these two coil systems each generating one magnetic field of mutually opposing directions along the z axis in the working volume. Utilization of the different polarities of the radially inner and radially outer coil system facilitates design of the magnet arrangement in accordance with the above-described embodiment.
The present invention also comprises a method for dimensioning a shim device which is characterized in that the variable gSeff, which corresponds to the field change in the working volume at z=0 per ampere current in the shim device, is calculated, taking into consideration the magnetic fields generated due to the currents induced in the residual magnet arrangement, according to:
gSeff=gSxe2x88x92gTxc2x7(Lclxe2x88x92xcex1Lcor)xe2x88x921xc2x7(L←Sclxe2x88x92xcex1L←Scor),
wherein these variables have the above-mentioned definitions. This method for dimensioning a shim device advantageously takes the magnetic shielding behavior of the superconductor in the magnet coil system into consideration. This method permits dimensioning of all embodiments of the invention through calculation of the behavior of the magnet system during charging of the shim device thereby taking into consideration the current changes induced in the magnet coil system and in the additional current paths. This method is based on the calculation of correction terms for the inductive couplings among the additional current paths, with the magnet coil system and with the shim device and for all self-inductances, which influence the corresponding quantities with a weighting factor xcex1. This method improves the correspondence between calculated and measurable field strength in the working volume gSeff of the shim device compared to a conventional method. In particular, gSeff can be made substantially zero.
In a simple variant of the inventive method, the parameter a corresponds to the volume portion of the superconductor material with respect to the coil volume of the magnet coil system. This method for determining the parameter xcex1 is based on the assumption that the superconductor has a susceptibility with respect to field fluctuations of (xe2x88x921) (ideal diamagnetism).
The values for xcex1 determined in this fashion cannot be experimentally confirmed for most magnet types. A particularly preferred alternative method variant is therefore characterized in that the parameter a for the magnet coil system is experimentally determined from the measurement of the variable       β    exp    =            g      D      exp              g      D      
of the magnet coil system, with no additional current paths present, in response to a disturbance coil generating a substantially homogeneous disturbance field in the volume of the magnet coil system, and through insertion of the variable xcex2exp into the equation       α    =                                                      g              D                        ⁡                          (                              L                M                cl                            )                                2                ⁢                  (                                    β              exp                        -                          β              cl                                )                                                                g              D                        ⁡                          (                                                β                  exp                                -                                  β                  cl                                            )                                ⁢                      L            M            cl                    ⁢                      L            M            cor                          -                              g            M                    ⁡                      (                                                            L                                      M                    ←                    D                                    cl                                ⁢                                  L                  M                  cor                                            -                                                L                                      M                    ←                    D                                    cor                                ⁢                                  L                  M                  cl                                                      )                                ,
wherein
gDexp: measured field change in the working volume of the magnet arrangement per ampere current in the disturbance coil,             β      cl        =          1      -                        g          M                ·                  (                                    L                              M                ←                D                            cl                                                      L                M                cl                            ·                              g                D                                              )                      ,
gM: Field per ampere of the magnet coil system in the working volume,
gD: Field per ampere of the disturbance coil in the working volume without the field contributions of the magnet coil system,
LMcl: Self inductance of the magnet coil system,
LM←Dcl: Inductive coupling of the disturbance coil with the magnet coil system,
LMcor: Correction for the self inductance LMcl of the magnet coil system, which would result for complete diamagnetic expulsion of disturbance fields from the volume of the magnet coil system,
LM←Dcor: Correction for the inductive coupling LM←Dcl of the disturbance coil with the magnet coil system which would result for complete diamagnetic expulsion of disturbance fields from the volume of the magnet coil system.
Finally, in a further particularly preferred variant of the inventive method, the corrections Lcor, L←Scor, LMcor and LM←Scor are calculated as follows:             L      cor        =          (                                                  L              M              cor                                                          L                              M                ←                P1                            cor                                            …                                              L                              M                ←                Pn                            cor                                                                          L                              P1                ←                M                            cor                                                          L              P1              cor                                            …                                              L                              P1                ←                Pn                            cor                                                            ⋮                                ⋮                                ⋰                                ⋮                                                              L                              Pn                ←                M                            cor                                                          L                              Pn                ←                P1                            cor                                            …                                              L              Pn              cor                                          )        ,      
    ⁢            L              ←        S            cor        =          (                                                  L                              M                ←                S                            cor                                                                          L                              P1                ←                S                            cor                                                            ⋮                                                              L                              Pn                ←                S                            cor                                          )        ,xe2x80x83LPj←Pkcor=fPj(LPj,red,Ra1)←Pkclxe2x88x92L(Pj,red,Ri1)←Pkcl),
LPj←Scor=fPj(LPj,red,Ra1)←Sclxe2x88x92L(Pj,red,Ri1)←Scl),
LPj←Mcor=fPj(LPj,red,Ra1)←Mclxe2x88x92L(Pj,red,Ri1)←Mcl),
            L              M        ←        Pj            cor        =                  L                  1          ←          Pj                cl            -              L                              (                          1              ,              red              ,                              Ri                1                                      )                    ←          Pj                cl            +                                    Ra            1                                R            2                          ⁢                  (                                    L                                                (                                      2                    ,                    red                    ,                                          Ra                      1                                                        )                                ←                Pj                            cl                        -                          L                                                (                                      2                    ,                    red                    ,                                          Ri                      1                                                        )                                ←                Pj                            cl                                )                      ,      
    ⁢            L              M        ←        S            cor        =                  L                  1          ←          S                cl            -              L                              (                          1              ,              red              ,              Ri1                        )                    ←          S                cl            +                                    Ra            1                                R            2                          ⁢                  (                                    L                                                (                                      2                    ,                    red                    ,                                          Ra                      1                                                        )                                ←                S                            cl                        -                          L                                                (                                      2                    ,                    red                    ,                                          Ri                      1                                                        )                                ←                S                            cl                                )                      ,      
    ⁢            L      M      cor        =                  L                  1          ←          1                cl            -              L                              (                          1              ,              red              ,              Ri1                        )                    ←          1                cl            +              L                  1          ←          2                cl            -              L                              (                          1              ,              red              ,              Ri1                        )                    ←          2                cl            +                                    Ra            1                                R            2                          ⁢                  (                                    L                                                (                                      2                    ,                    red                    ,                                          Ra                      1                                                        )                                ←                2                            cl                        -                          L                                                (                                      2                    ,                    red                    ,                                          Ri                      1                                                        )                                ←                2                            cl                        +                          L                                                (                                      2                    ,                    red                    ,                                          Ra                      1                                                        )                                ←                1                            cl                        -                          L                                                (                                      2                    ,                    red                    ,                                          Ri                      1                                                        )                                ←                1                            cl                                )                    
wherein
Ra1: Outer radius of the magnet coil system (in case of an actively shielded magnet coil system, the outer radius of the main coil),
Ri1: Inner radius of the magnet coil system,
R2: in the case of an actively shielded magnet coil system, the average radius of the shielding, otherwise infinite,
RPj: average radius of the additional coil Pj,       f    Pj    =      {                                                                      Ra                1                                            R                Pj                                      ,                                          R                Pj                             greater than                               Ra                1                                                                                      1            ,                                          R                Pj                             less than                               Ra                1                                                        
and wherein the index 1 represents the main coil of an actively shielded magnet coil system, otherwise the magnet coil system, and the index 2 represents the shielding of an actively shielded magnet coil system, and otherwise terms of index 2 are omitted, and the index (X,red,R) designates a hypothetical coil having all windings of a coil X at the radius R.
The particular advantage of this method for calculating the corrections Lcor, L←Scor, LMcor and LM←Scor consists in that the corrections are derived from inductive couplings and self-inductances of coils thereby taking into consideration the geometric arrangement of the coils concerned.
Further advantages of the invention can be extracted from the description and the drawing. The features mentioned above and below can be used in accordance with the invention either individually or collectively in any arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration but rather have exemplary character for describing the invention.
The invention is shown in the drawing and explained in more detail with respect to embodiments.